Liquid crystal display device having rectangular-shaped pixel electrodes overlapping with comb-shaped counter electrodes in plan view

ABSTRACT

The present invention realizes a bright image display by enhancing a numerical aperture of pixels. At least a portion of a pixel electrode is overlapped to a thin film transistor by way of a first insulation film, the pixel electrode is connected to an output electrode of the thin film transistor via a contact hole which is formed in the first insulation film, the counter electrode is arranged above the pixel electrode by way of a second insulation film in a state that the counter electrode is overlapped to the pixel electrode, the counter electrode is formed at a position avoiding the contact hole formed in the first insulation film as viewed in a plan view, and at least a portion of the counter electrode is overlapped to the thin film transistor.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation application of U.S. application Ser.No. 11/498,158 filed on Aug. 3, 2006 now U.S. Pat. No. 7,605,898. Thepresent application claims priority from U.S. application Ser. No.11/498,158 filed on Aug. 3, 2006, which claims priority from JapanesePatent Application No. 2005-278161 filed on Sep. 26, 2005, the contentof which is hereby incorporated by reference into this application.

BACKGROUND OF THE INVENTION

The present invention relates to a liquid crystal display device, andmore particularly to a lateral-electric-field-type liquid crystaldisplay device which can enhance a display numerical aperture.

As a display device for various kinds of personal digital assistants andtelevision receiver sets, a display device which uses a so-calledflat-type display panel as represented by a liquid crystal displaydevice has been a mainstream. Further, as the liquid crystal displaydevice, an active-method-type liquid crystal display device has beenpopularly used. The active-method-type liquid crystal display devicegenerally uses a thin film transistor as a drive element of a pixel andhence, hereinafter, the display device which adopts the thin filmtransistor as the drive element is explained as an example. As one typeof such a liquid crystal display device, there has been known alateral-electric-field-type liquid crystal display device which isreferred to IPS (in-plane-switching) type display device.

FIG. 9 is a plan view for explaining an example of the base pixelstructure of the thin film transistor of the IPS type liquid crystaldisplay device. Further, FIG. 10 is a cross-sectional view of thedisplay device which also includes a color filter substrate taken alonga line D-D′ in FIG. 9. The planer constitution of the pixel of the IPStype liquid crystal display device is, as shown in FIG. 9, formed in theinside of a region which is surrounded by two gate lines GL and two datalines DL. A thin film transistor TFT is formed on a portion of theregion (pixel region). The thin film transistor TFT has a drain (or asource) electrode SD2 thereof connected to the data line DL, has a gateelectrode GT thereof connected to the gate line GL, and has a source(drain) electrode SD1 connected to a pixel electrode PX through acontact hole CH. Here, although the drain electrode and the sourceelectrode are exchanged from each other during an operation, theexplanation is made hereinafter with respect to a case in which the thinfilm transistor TFT includes the source electrode SD1 and the drainelectrode SD2.

As shown in FIG. 10, the cross-sectional structure of the pixel formsthe thin film transistor TFT which is constituted of a semiconductorlayer (silicon semiconductor) SI, a second insulation film INS2, thegate electrode GT, a third insulation film INS3, the source electrodeSD1 and the drain electrode SD2 on a first insulation film INS1 which isformed on a main surface of one substrate (a thin film transistorsubstrate, hereinafter, a TFT substrate) SUB1 which is preferably madeof glass. Here, the scanning lines GL shown in FIG. 9 are formed on thesame layer as the gate electrodes GT, the data lines DL are formed onthe third insulation film INS3, and the source electrodes SD1 and thedrain electrodes SD2 are formed on the same layer as the data lines DL.The source electrodes SD1 and the drain electrodes SD2 are connected tothe semiconductor layers SI via the contact holes which are formed inthe second insulation film INS2 at the time of forming the sourceelectrodes SD1, drain electrodes SD2 as films.

A fourth insulation film INS4 which constitutes a protective film(passivation film) is formed in a state that the fourth insulation filmINS4 covers the source electrode SD1, the drain electrode SD2 and thedata lines DL. Here, a counter electrode CT is formed in a spreadingmanner on the fourth insulation film INS4 in a state that a contactelectrode CT covers a most portion of the pixel region, and a contacthole CH which reaches the source electrode SD1 is formed in the fourthinsulation film INS4. Further, a fifth insulation film INS5 is formed ina state that the fifth insulation film INS5 covers the counter electrodeCT.

The pixel electrode PX is formed on the fifth insulation film INS5 in acomb-teeth shape, and one end of the pixel electrode PX is connected tothe source electrode SD1 via the contact hole CH. Then, an orientationfilm ORI1 is formed in a state that the orientation film covers atopmost surface of the pixel electrode PX.

On a main surface of another substrate (color filter substrate,hereinafter, referred to as a CF substrate) SUB2 which is preferablymade of glass, color filters CF which are defined from each other by ablack matrix BM are formed, and an orientation film ORI2 is formed on atopmost surface of the substrate SUB2. The current display devicesmostly adopt a full color display. In this full color display,basically, unit pixels (sub pixels) of three colors consisting of red(R), green (G), and blue (B) constitute one color pixel.

In the IPS type liquid crystal display device, a liquid crystal LC issealed in the inside of a space between the orientation film ORI1 of theTFT substrate SUB1 and the orientation film ORI2 of the CF substrateSUB2. The liquid crystal LC which is driven by the thin film transistorTFT is rotated by a component of an electrical field E parallel to asurface of the substrate which is generated between the pixel electrodePX and the counter electrode CT in the inside of the surface in whichthe orientation direction of the liquid crystal LC is parallel to thesurface of the substrate and hence, the lighting and non-lighting of thepixel can be controlled. As a document which discloses such an IPS-typeliquid crystal display device, International Publication WO 01/018597can be named.

SUMMARY OF THE INVENTION

In an IPS-type liquid crystal display device, as shown in FIG. 10, aportion of a contact hole CH which connects a pixel electrode to asource electrode constituting an output electrode of the thin filmtransistor is not used as a display region together with a portion onwhich the thin film transistor is arranged. Also a black matrix which ismounted on a CF substrate is formed in a state that the black matrixcovers the thin film transistor and the contact hole portion.Accordingly, the increase of an effective area of the pixel, that is,the enhancement of a numerical aperture is limited.

It is an object of the present invention to provide alateral-electric-field-type liquid crystal display device which canenhance a numerical aperture of pixels thus realizing a bright imagedisplay.

Typical constitutions of the present invention are describedhereinafter.

(1) A liquid crystal display device which includes a first substratehaving pixel electrodes, counter electrodes and thin film transistors, asecond substrate which faces the first substrate in an opposed manner,and a liquid crystal layer between the first substrate and the secondsubstrate, wherein

at least a portion of the pixel electrode is overlapped to the thin filmtransistor via a first insulation film, the pixel electrode is connectedto an output electrode of the thin film transistor via a contact holewhich is formed in the first insulation film, and

the counter electrode is arranged above the pixel electrode by way of asecond insulation film in a state that the counter electrode isoverlapped to the pixel electrode, the counter electrode is formed at aposition avoiding the contact hole formed in the first insulation filmas viewed in a plan view, and at least a portion of the counterelectrode is overlapped to the thin film transistor.

(2) In the constitution (1), a region where the contact hole is formedis a reflective display region.

(3) In the constitution (1) or (2), at least a portion of the region ofthe thin film transistor constitutes a reflective display region.

(4) In the constitution (1), the liquid crystal display device includesa reflective display region.

(5) In any one of the constitutions (1) to (4), the liquid crystaldisplay device includes a reflective display region and a transmissivedisplay region.

(6) In any one of the constitutions (1) to (5), an input electrode andthe output electrode of the thin film transistor are formed of areflective conductive film.

(7) In any one of the constitutions (1) to (6), the pixel electrode isformed of a transparent conductive film.

(8) In any one of the constitutions (1) to (6), at least a portion ofthe pixel electrode is formed of a reflective conductive film.

(9) In any one of the constitutions (1) to (4), the pixel electrode, theinput electrode and the output electrode of the thin film transistor areformed of a reflective conductive film.

(10) In any one of the constitutions (1) to (9), a portion of the secondinsulation film is filled in the contact hole, and the second insulationfilm in the region overlapped to the contact hole has aliquid-crystal-layer-side surface thereof leveled.

(11) In any one of the constitutions (1) to (9), an insulation memberwhich is made of a material different from a material of the secondinsulation film is filled in the contact hole, and the second insulationfilm in the region overlapped to the contact hole has aliquid-crystal-layer-side surface thereof leveled.

It is needless to say that the present invention is not limited to theconstitutions which are described above and the constitutions which willbe explained in conjunction with embodiments described later and variousmodifications are conceivable without departing from the technicalconcept of the present invention.

According to the constitution of the present invention, the thin filmtransistor and the contact hole portion which connects an outputelectrode of the TFT to the pixel electrode which have been consideredas portions which do not contribute to a display in the pixel region canbe used as the reflective display region and hence, it is possible toenhance a numerical aperture and to increase a display brightness.Further, conventionally, thicknesses of the electrode and the insulationfilm which have been formed on the contact hole portion are reducedcompared to thicknesses of the electrode and the insulation film whichhave been formed on a leveled surface portion and hence, there exists apossibility that short-circuiting occurs between the counter electrodeand the pixel electrode which are formed on the contact hole portion.According to the present invention which eliminates the mounting of thecounter electrode on the contact hole portion, it is possible to avoidthe occurrence of the above-mentioned short-circuiting.

Further, according to the present invention, since the surface of theinsulation film can be leveled by embedding the insulation member in theopening formed in the insulation film which is formed on the contacthole portion, the orientation film which is formed over the insulationfilm can be leveled. Accordingly, it is also possible to impart theaccurate orientation to the liquid crystal on the contact hole portionin the same manner as other portions thus eliminating a display defectsuch as a light leakage or the like attributed to the defectiveorientation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view which shows an example of the constitution of onepixel for explaining an embodiment 1 of a liquid crystal display deviceaccording to the present invention;

FIG. 2 is a cross-sectional view taken along a line A-A′ in FIG. 1;

FIG. 3 is a plan view which shows an example of the constitution of onepixel for explaining an embodiment 2 of a liquid crystal display deviceaccording to the present invention;

FIG. 4 is a cross-sectional view taken along a line B-B′ in FIG. 3;

FIG. 5 is a plan view which shows an example of the constitution of onepixel for explaining an embodiment 3 of a liquid crystal display deviceaccording to the present invention;

FIG. 6 is a cross-sectional view taken along a line C-C′ in FIG. 5;

FIG. 7 is an explanatory view of a transmissive brightness-voltagecharacteristic due to a dielectric constant of a fifth insulation filmfor explaining an embodiment 3;

FIG. 8 is an explanatory view of a transmissive brightness-voltagecharacteristic due to a film thickness of the fifth insulation film forexplaining the embodiment 3;

FIG. 9 is a plan view which explains one example of the basic pixelstructure of a thin film transistor-substrate-side of an ISP-type liquidcrystal display device; and

FIG. 10 is a cross-sectional view of the liquid crystal display devicewhich also includes a color filter substrate taken along a line D-D′ inFIG. 9.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, embodiments of the present invention are explained indetail in conjunction with drawings of embodiments.

Embodiment 1

FIG. 1 is a plan view which shows an example of the constitution of onepixel for explaining an embodiment 1 of a liquid crystal display deviceaccording to the present invention. Further, FIG. 2 is a cross-sectionalview taken along a line A-A′ in FIG. 1. This liquid crystal displaydevice is of an IPS type. In the same manner as the display device shownin FIG. 10, a pixel region is formed in a region which is surrounded bytwo scanning lines (hereinafter, also referred to as gate lines) GL andtwo image signal lines (hereinafter, also referred to as data lines) DL.A thin film transistor TFT which constitutes an active element is formedin a portion of the pixel region. The thin film transistor TFT has adrain (or a source) electrode SD2 thereof connected to the data line DL,has a gate electrode GT thereof connected to the gate line GL and has asource (a drain) electrode SD1 thereof connected to a pixel electrode PXvia a contact hole CH.

As shown in FIG. 2 which is a cross-sectional view taken along a lineA-A′ in FIG. 1, the cross-sectional structure of the pixel includes thethin film transistor TFT which is constituted of a semiconductor layer(silicon semiconductor) SI, a second insulation film INS2, the gateelectrode GT, a third insulation film INS3, the source electrode SD1 andthe drain electrode SD2 on a first insulation film INS1 which is formedon a main surface of one substrate (thin film transistor substrate,hereinafter, also referred to as a TFT substrate) SUB1 which ispreferably made of glass. Here, the gate lines GL shown in FIG. 1 areformed on the same layer as the gate electrodes GT, the data lines DLare formed on the third insulation film INS3, and the source electrodesSD1 and the drain electrodes SD2 are formed on the same layer as thedata lines DL. The source electrode SD1 and the drain electrode SD2 areconnected to the semiconductor layer SI via contact holes which areformed in the second insulation film INS2 at the time of forming theseelectrodes.

A fourth insulation film INS4 which constitutes a protective film(passivation film) is formed in a state that the fourth insulation filmINS4 covers the source electrode SD1, the drain electrode SD2 and thedata lines DL. Here, a pixel electrode PX is formed in a spreadingmanner on the fourth insulation film INS4 in a state that the pixelelectrode PX covers a most portion of the pixel region including aportion above the thin film transistor TFT. A contact hole CH whichreaches the source electrode SD1 is formed in the fourth insulation filmINS4. Further, a fifth insulation film INS5 is formed on the fourthinsulation film INS4 in a state that the fifth insulation film INS5covers the pixel electrode PX. A counter electrode CT is formed on thefifth insulation film INS5 in a comb-teeth shape. Here, symbol PEindicates cutout portions of the counter electrode CT and the pixelelectrode which is exposed from the cutout portions are viewed. Further,an orientation film ORI1 is formed to cover a topmost surface of thecounter electrode CT.

On the main surface of another substrate (color filter substrate,hereinafter, referred to as a CF substrate) SUB2 which is preferablymade of glass, the color filters CF which are defined from each other bya black matrix BM are formed, and an orientation film ORI2 is formed ona topmost surface of the substrate SUB2. The currently available displaydevices mostly adopt a full color display. In the full color display(hereinafter, also simply referred to as a color display), basically,unit pixels (sub pixels) of three colors consisting of red (R), green(G), and blue (B) constitute one color pixel.

In the IPS-type liquid crystal display device, liquid crystal LC issealed in the inside of a space between the orientation film ORI1 of theTFT substrate SUB1 and the orientation film ORI2 of the CF substrateSUB2. With respect to the liquid crystal LC which is driven by the thinfilm transistor TFT, the orientation direction of the liquid crystal LCis rotated by a component parallel to a surface of the substrate of anelectrical field E which is generated between the pixel electrode PX andthe counter electrode CT in a plane parallel to the substrate surfacethus controlling the lighting and non-lighting of the pixel.

Here, the manufacturing process of the liquid crystal display device ofthe embodiment 1 is explained. On an insulation substrate which ispreferably made of glass, a semiconductor island is formed by forming ana-Si or p-Si semiconductor film by patterning. Since a process forforming the insulation films and the gate electrodes and a process forforming the source electrodes SD1 and the drain electrodes SD2 on thesemiconductor island are already known, the explanation of theseprocesses is omitted. In the embodiment 1, the source electrode SD1 andthe drain electrode SD2 of the thin film transistor TFT are formed of astacked film of MoW/AlSi/MoW.

After forming the source electrode SD1 and the drain electrode SD2, afourth insulation film is formed over the source electrode SD1 and thedrain electrode SD2. A forming method of the fourth insulation film isexplained hereinafter. First of all, an organic resin which is formed ofpolymethyl silazane is applied to the substrate using a spin coatingmethod. Using a photo mask which has a desired pattern, the exposure isperformed by radiating i rays to the organic resin and, thereafter, theorganic resin is humidified thus forming silanol. The silanol isdeveloped by an alkali developer and is removed. Next, the full surfaceexposure is performed by radiating ghi rays to the substrate and,thereafter, the substrate is humidified again. Accordingly, the silanolis formed on a portion where the silanol is not removed by theabove-mentioned developing. Polymethyl siloxane is formed on the desiredportion by baking the silanol thus forming the fourth insulation film.

A contact hole which connects the source electrode of the thin filmtransistor TFT and the pixel electrode described later to each other isformed by removing the insulation film 4 by patterning. A thickness ofthe insulation film 4 is set to 1 μm.

With respect to the pixel electrode PX, an ITO film which is atransparent conductor film is formed with a thickness of 77 nm bysputtering, and a photosensitive resist is applied to the ITO film. Theexposure is performed using a photo mask which has a desired pattern,and the photosensitive resist is partially removed using an alkalideveloper (the exposed portion being removed when a positive-typephotosensitive resist is used). Using the pattern of the photosensitiveresist as a mask, the transparent conductor film is removed by an ITOetchant (for example, oxalic acid).

Then, the photosensitive resist is removed using a resist peeling liquid(for example, monoethanolamine: MEA). The pattern of the formed pixelelectrode PX has a rectangular shape, and is formed on the substantiallywhole surface of the region which is surrounded by the image signallines and the scanning signal lines.

On the ITO film which constitutes the pixel electrode PX, a fifthinsulation film INS5 which is made of SiN (dielectric constant: 6.7) isformed using a CVD method. In this embodiment, a thickness of the fifthinsulation film is set to 300 nm. Here, although the patterning of thefifth insulation film is substantially equal to the patterning adoptedby a method for forming the pixel electrode, the SiN film is etched bydry etching using a SF₆+O₂ gas or a CF₄ gas.

The comb-teeth-shaped counter electrode CT is formed in the same processas the pixel electrode PX. The counter electrode CT is formed byavoiding a portion above the contact hole which connects the pixelelectrode PX and the source electrode of the thin film transistor TFT toeach other.

Next, a driving method of the liquid crystal display device of theembodiment 1 is explained. An image signal is supplied to the pixelelectrode PX via the thin film transistor TFT. A constant voltage isapplied to the counter electrode CT or an AC voltage (AC driving) isapplied to the counter electrode at the timing of supplying scanningsignals. When such a voltage is applied, between the pixel electrode PXand the edge of the comb-teeth shaped counter electrode CT, a so-calledfringe electric field E is generated (see, FIG. 1). Further, themolecular orientation of the liquid crystal LC is controlled by thefringe electric field E.

In the embodiment 1, since the counter electrode CT is not arrangedabove the contact hole for connecting the pixel electrode PX to thesource electrode of the thin film transistor TFT, the liquid crystalmolecules which exist above the contact hole have the orientationthereof also controlled by the fringe electric field E and contribute toa display. That is, by forming the source electrode SD1 and the drainelectrode SD2 using a reflective conductive film, an upper region of thethin film transistor TFT which includes the contact hole CH portion alsoforms a reflective display region, while forming the pixel electrode PXin the pixel region other than the thin film transistor TFT as thetransparent conductive film such as the ITO film, it is possible toconstitute a reflective/transmissive liquid crystal display device whichcan enhance an numerical aperture thereof. Further, even when a portionof the pixel electrode PX is formed of the reflective conductive film,it is possible to constitute the reflective/transmissive liquid crystaldisplay device.

Further, by forming a reflective metal film on the ITO film whichconstitutes the pixel electrode PX or by forming the whole pixelelectrode PX per se using a reflective conductive film in the samemanner as the source electrode SD1 and the drain electrode SD2, it ispossible to constitute a reflective liquid crystal display device.

Still further, by adopting the constitution described in the embodiment1, even when the insulation film at the contact hole CH portion has asmall thickness, the counter electrode CT is not arranged on the portionand hence, the occurrence of the short-circuiting of the pixel electrodePX and the counter electrode CT is prevented whereby a yield rate isenhanced thus enabling the acquisition of a highly reliable liquidcrystal display device.

Embodiment 2

FIG. 3 is a plan view which shows an example of the constitution of onepixel for explaining an embodiment 2 of a liquid crystal display deviceaccording to the present invention, while FIG. 4 is a cross-sectionalview taken along a line B-B′ in FIG. 3. The liquid crystal displaydevice of the embodiment 2 is also of an IPS type. In the same manner asthe embodiment 1, a pixel region is formed in the inside of a regionwhich is surrounded by two gate lines GL and two data lines DL. A thinfilm transistor TFT is formed at a portion of the pixel region. The thinfilm transistor TFT has a drain (or a source) electrode SD2 thereofconnected to the data line DL, has a gate electrode GT thereof connectedto the gate line GL, and has a source (a drain) electrode SD1 thereofconnected to a pixel electrode PX via a contact hole CH.

As shown in FIG. 4 which is a cross-sectional view taken along a lineB-B′ in FIG. 3, the cross-sectional structure of the pixel includes thethin film transistor TFT which is constituted of a semiconductor layerS1, a second insulation film INS2, the gate electrode GT, a thirdinsulation film INS3, the source electrode SD1 and the drain electrodeSD2 on a first insulation film INS1 which is formed on a main surface ofa TFT substrate SUB1. Here, the gate lines GL shown in FIG. 3 are formedon the same layer as the gate electrodes GT, the data lines DL areformed on the third insulation film INS3, and the source electrodes SD1and the drain electrodes SD2 are formed on the same layer as the datalines DL. The source electrode SD1 and the drain electrode SD2 areconnected to the semiconductor layer SI via contact holes which areformed in the second insulation film INS2 at the time of forming theseelectrodes.

A fourth insulation film INS4 is formed in a state that the fourthinsulation film INS4 covers the source electrode SD1, the drainelectrode SD2 and the data lines DL. Here, a pixel electrode PX isformed in a spreading manner on the fourth insulation film INS4 in astate that the pixel electrode PX covers a most portion of the pixelregion including a portion above the thin film transistor TFT. A contacthole CH which reaches the source electrode SD1 is formed in the fourthinsulation film INS4. Further, a fifth insulation film INS5 is formed ina state that the fifth insulation film INS5 covers the pixel electrodePX. The fifth insulation film INS5 is filled in the inside of thecontact hole CH, and a surface of the fifth insulation film INS5including an upper portion of the contact hole CH is leveled. On theleveled fifth insulation film INS5, a counter electrode CT is formed ina comb-teeth shape. Here, symbol PE indicates cutout portions of thecounter electrode CT, and the pixel electrode which is exposed from thecutout portions are viewed. Further, an orientation film ORI1 is formedto cover a topmost surface of the counter electrode CT.

On a main surface of a CF substrate SUB2, color filters CF which aredefined from each other by a black matrix BM are formed in the samemanner as the embodiment 1, and an orientation film ORI2 is formed on atopmost surface of the CF substrate SUB2. Each color filter CF isbasically constituted of unit pixels (sub pixels) of three colorsconsisting of red (R), green (G), and blue (B), and a color one pixel(pixel) is constituted of the three colors of the unit pixels.

Liquid crystal LC is sealed in the inside of a space between theorientation film ORI1 of the TFT substrate SUB1 and the orientation filmORI2 of the CF substrate SUB2. With respect to the liquid crystal LCwhich is driven by the thin film transistor TFT, the orientationdirection of the liquid crystal is rotated by a component parallel to asurface of the substrate of an electrical field E which is generatedbetween the pixel electrode PX and the counter electrode CT in a planeparallel to the substrate surface thus controlling the lighting andnon-lighting of the pixel.

The manufacturing process of the liquid crystal display device of theembodiment 2 is explained hereinafter by focusing on points which makethis embodiment 2 different from the embodiment 1. With respect to theprocess up to the formation of the pixel electrode PX in which thesource electrode SD1 and the drain electrode SD2 are formed, the fourthinsulation film is formed over the source electrode SD1 and the drainelectrode SD2 and, thereafter, the pixel electrodes PX are formed, sucha process is substantially equal to the process of the embodiment 1.Thereafter, a photosensitive resist (for example, JSR-made PC-452) isapplied to an ITO film which forms the pixel electrodes PX. Thephotosensitive resist is exposed using a photo mask which has a desiredpattern, the photosensitive resist is partially removed using an alkalideveloper, and the stacked structure is baked. Although irregularitiesof a surface can be controlled based on baking conditions of thisbaking, in this embodiment, a baking temperature is set to 230° C. and abaking period is set to 60 minutes so as to substantially level asurface of the fifth insulation film INS5. Further, the fifth insulationfilm INS5 assumes a film thickness of 300 nm at a surface leveledportion (other than the contact hole portion) of the pixel electrodesafter baking.

Although a forming process of the counter electrode CT of the embodiment2 is equal to the forming process of the counter electrode CT of theembodiment 1, in the embodiment 2, the formation of the counterelectrode CT above the contact hole CH is not excluded.

Next, a driving method of the liquid crystal display device of theembodiment 2 is explained hereinafter by focusing on points which makethis embodiment 2 different from the embodiment 1. In the embodiment 1,rubbing treatment of the orientation film may not be sufficientlyperformed depending on the degree of irregularities of a portion onwhich the contact hole CH is formed and hence, a liquid crystalorientation regulating force (an anchoring strength) may become smallthus easily generating an image retention. The image retention is aphenomenon in which liquid crystal which is driven by an electric fielddoes not return to an initial state even after the electric field iseliminated. However, according to the constitution of the embodiment 2,the surface of the fifth insulation film INS5 which is formed above aportion in which the contact hole CH is formed is leveled and hence, therubbing treatment can be performed sufficiently whereby the generationof the image retention can be suppressed.

Embodiment 3

FIG. 5 is a plan view which shows an example of the structure of onepixel for explaining an embodiment 3 of the liquid crystal displaydevice according to the present invention, while FIG. 6 is across-sectional view taken along a line C-C′ in FIG. 5. The liquidcrystal display device of the embodiment 3 is also of an IPS type. Inthe same manner as the embodiment 1 and the embodiment 2, a pixel regionis formed in the inside of a region which is surrounded by two gatelines GL and two data lines DL. A thin film transistor TFT is formed ata portion of the pixel region. The thin film transistor TFT has a drain(or a source) electrode SD2 thereof connected to the data lines DL, hasa gate electrode GT thereof connected to the gate lines GL and has asource (a drain) electrode SD1 there of connected to a pixel electrodePX via a contact hole CH.

As shown in FIG. 6 which is a cross-sectional view taken along a lineC-C′ in FIG. 5, the cross-sectional structure of the pixel includes thethin film transistor TFT which is constituted of a semiconductor layerS1, a second insulation film INS2, the gate electrode GT, a thirdinsulation film INS3, the source electrode SD1 and the drain electrodeSD2 on a first insulation film INS1 which is formed on a main surface ofa TFT substrate SUB1. Here, the gate lines GL shown in FIG. 5 are formedon the same layer as the gate electrodes GT, the data lines DL areformed on the third insulation film INS3, and the source electrodes SD1and the drain electrodes SD2 are formed on the same layer as the datalines DL. The source electrode SD1 and the drain electrode SD2 areconnected to the semiconductor layer SI via contact holes which areformed in the second insulation film INS2 at the time of forming theseelectrodes.

A fourth insulation film INS4 is formed in a state that the fourthinsulation film INS4 covers the source electrode SD1, the drainelectrode SD2 and the data lines DL. Here, a pixel electrode PX isformed in a spreading manner on the fourth insulation film INS4 in astate that the pixel electrode PX covers a most portion of the pixelregion including a portion above the thin film transistor TFT. A contacthole CH which reaches the source electrode SD1 is formed in the fourthinsulation film INS4. A sixth insulation film INS6 is filled in theinside of the contact hole CH, a fifth insulation film INS5 is formed onthe sixth insulation film INS6 in a state that the fifth insulation filmINS5 covers the pixel electrode PX. A surface of the fifth insulationfilm INS5 including an upper portion of the contact hole CH is leveled.On the leveled fifth insulation film INS5, a counter electrode CT isformed in a comb-teeth shape. Here, symbol PE indicates cutout portionsof the counter electrode CT, and the pixel electrode which is exposedfrom the cutout portions are viewed. Further, an orientation film ORI1is formed to cover a topmost surface of the counter electrode CT.

On a main surface of a CF substrate SUB2, color filters CF which aredefined from each other by a black matrix BM are formed in the samemanner as the embodiment 1 and the embodiment 2, and an orientation filmORI2 is formed on a topmost surface of the CF substrate SUB2. Each colorfilter CF is basically constituted of unit pixels (sub pixels) of threecolors consisting of red (R), green (G), and blue (B), and a color pixel(pixel) is constituted of the unit pixels of the three colors.

A liquid crystal LC is sealed in the inside of a space between theorientation film ORI1 of the TFT substrate SUB1 and the orientation filmORI2 of the CF substrate SUB2. With respect to the liquid crystal LCwhich is driven by the thin film transistor TFT, the orientationdirection of the liquid crystal is rotated by a component parallel to asurface of the substrate of an electrical field E which is generatedbetween the pixel electrode PX and the counter electrode CT in a planeparallel to the substrate surface thus controlling the lighting andnon-lighting of the pixel.

In the embodiment 3, A SiN film having a film thickness of 300 nm isformed as the fifth insulation film INS5. The patterning of the fifthinsulation film INS5 is substantially equal to the patterning of thefifth insulation film adopted in the embodiment 1. After the formationof the fifth insulation film INS5, the photosensitive resist (PC-452made by JSR) is filled in the contact hole CH thus forming sixthinsulation film INS6. The patterning of the photosensitive resist issubstantially equal to the patterning of the photosensitive resistadopted in the embodiment 2. The forming process of the counterelectrode CT in the embodiment 3 is substantially equal to the formingprocess of the counter electrode CT adopted in the embodiments 1, 2.

A driving method of the liquid crystal display device of the embodiment3 is explained hereinafter by focusing on points which make thisembodiment 3 different from the embodiment 2. In the embodiment 3, theinsulation film which is arranged between the pixel electrode PX and thecounter electrode CT exhibits a high dielectric constant, and thesmaller the film thickness of the insulation film, an electric fieldapplied to the liquid crystal is increased thus eventually lowering aliquid crystal drive voltage (see FIG. 7, FIG. 8).

FIG. 7 is an explanatory view of a transmissive brightness-voltagecharacteristic due to a dielectric constant of a fifth insulation filmfor explaining the embodiment 3, while FIG. 8 is an explanatory view ofa transmissive brightness-voltage characteristic due to a film thicknessof a fifth insulation film for explaining the embodiment 3. In FIG. 7and FIG. 8, symbol V indicates a liquid crystal drive voltage, symbol Ttindicates transmissivity, ∈5 indicates the dielectric constant of thefifth insulation film INS5, and symbol t5 indicates a film thickness ofthe fifth insulation film INS5. Here, a width W of the teeth-likecounter electrode CT and an electrode distance L of the counterelectrodes CT is 4 μm and 6 μm respectively.

In the embodiment 2, a film thickness of the resin having a dielectricconstant of 3.3 which constitutes the fifth insulation film INS5 may beset to a value less than 300 nm. However, this resin exhibits a poorinsulation dielectric strength and hence, a leak current is generatedbetween the pixel electrode and the counter electrode thus lowering aliquid holding voltage. To the contrary, in the embodiment 3, at aportion where the pixel electrode is leveled, by forming the fifthinsulation film INS5 using SiN having a high dielectric constant and ahigh insulation dielectric strength (dielectric constant: 6.7), it ispossible to realize the low drive voltage and the suppression of leakcurrent. Further, at a portion which has the irregular surface such asthe contact hole, by adopting the stacked structure consisting of thefifth insulation film INS5 and the sixth insulation film INS6, it ispossible to prevent the short-circuiting between the pixel electrode andthe counter electrode and hence, the surface of the fifth insulationfilm INS5 can be leveled whereby it is possible to realize theenhancement of the liquid crystal orientation restricting force.

1. A liquid crystal display device comprising: a first substrate whichforms pixel electrodes, counter electrodes, thin film transistors andpixel regions thereof; a second substrate which faces the firstsubstrate in an opposed manner; and a liquid crystal layer providedbetween the first substrate and the second substrate, wherein each ofthe pixel regions is surrounded by a pair of gate lines and a pair ofdrain lines, each of the pixel electrodes is formed in a correspondingone of the pixel regions respectively, each of the pixel electrodes isformed in a rectangular shape which substantially covers saidcorresponding pixel regions in a plan view, each of the counterelectrode is formed in a comb shape in a plan view and has teethportions, a display region which contributes to a display is formed insaid each pixel region, at least a portion of said each pixel electrodeoverlaps a corresponding one of the thin film transistors in a plan viewvia a first insulation film provided therebetween, said each pixelelectrode is connected to an output electrode of the thin filmtransistor via a contact hole which is formed in the first insulationfilm, the contact hole being disposed in the display region formed insaid corresponding pixel region and covered with said each pixelelectrode, said each counter electrode is arranged at a portion above anentire portion of said each pixel electrode with a second insulationfilm in-between in an area where the teeth portions of said each counterelectrode overlap said each pixel electrode in the plan view, and atleast one of the teeth portions of said each counter electrode overlapsa semiconductor layer of the thin film transistor in the plan view.
 2. Aliquid crystal display device according to claim 1, wherein at least aportion of the semiconductor layer of the thin film transistor isdisposed in the display region.
 3. A liquid crystal display deviceaccording to claim 1, wherein said each counter electrode is formed at aposition avoiding the contact hole formed in the first insulation filmin the plan view.
 4. A liquid crystal display device according to claim1, wherein a portion of the second insulation film is filled in thecontact hole.
 5. A liquid crystal display device according to claim 4,wherein the second insulation film in a region overlapped to the contacthole has a liquid-crystal-layer-side surface thereof leveled.
 6. Aliquid crystal display device according to claim 4, wherein at least oneof the teeth portions of said each counter electrode overlaps thecontact hole in the plan view.
 7. A liquid crystal display deviceaccording to claim 1, wherein an insulation member which is made of amaterial different from a material of the second insulation film isfilled in the contact hole.
 8. A liquid crystal display device accordingto claim 7, wherein the second insulation film in a region overlapped tothe contact hole has a liquid-crystal-layer-side surface thereofleveled.
 9. A liquid crystal display device according to claim 7,wherein the insulation member is made of a photosensitive resist.
 10. Aliquid crystal display device according to claim 7, wherein at least oneof the teeth portions of said each counter electrode overlaps thecontact hole in the plan view.
 11. A liquid crystal display deviceaccording to claim 1, wherein the display region where the contact holeis disposed is a reflective display region.
 12. A liquid crystal displaydevice according to claim 1, wherein at least a portion of a regionwhere the thin film transistor is disposed constitutes a reflectivedisplay region.
 13. A liquid crystal display device according to claim1, wherein the liquid crystal display device includes a reflectivedisplay region and a transmissive display region.